The present invention relates generally to an inverter control or protection system and more particularly to a system for the detection and correction of a conduction-through fault.
Conduction-through faults or "shoot-throughs" have been a problem in inverters for many years. A conduction-through fault is, very briefly, the failure of one or more thyristors in the inverter to commutate or achieve forward blocking at the end of its normal period of conduction. This failure to achieve forward blocking enables direct current to continue through the thyristor when the thyristor should be in the off state. A conduction-through fault may be caused by an inadequate commutation margin due to, for example, suppression or absence of gating, gating too late, excessive load current, severe line dip, and perhaps other circumstances.
The traditional approach in line commutated inverters (including its subspecies type sometimes referred to as a load commutated inverter) has been to prevent or reduce the chances of a conduction-through fault occurring by maintaining an adequate commutation angle as far as possible, and if a conduction-through fault should occur, to interrupt the fault current with a high speed circuit breaker or fuse. It is, however, desirable to attempt to recover from a conduction-through fault without shutting down or causing damage to the inverter.
In forced commutation type inverters, arrangements have been developed which may permit recovery from a conduction-through fault. Canadian Pat. No. 998,739 to Brenneisen et al, issued Oct. 19, 1976, describes such an arrangement. In this arrangement, when an overload in one branch is detected, all the thyristors are gated on thereby splitting the overload and reducing the current in the leg where the overload occurred. The inverter has inductive coils in each leg and an inductance in parallel with a capacitance across the input. The values are selected to provide a tuned circuit which is responsive to the current to cause an oscillatory current with a period twice as long as the maximum conductive period of a thyristor and an amplitude larger than the short circuit current. The oscillatory current will oppose the overload current and quench the thyristors.
The aforementioned arrangement is not readily applicable to line commutated inverters with which the present invention is concerned and is not economically attractive.